[DAG] fold negation of sign-bit
0 - X --> 0, if the sub is NUW
0 - X --> 0, if X is 0 or the minimum signed value and the sub is NSW
0 - X --> X, if X is 0 or the minimum signed value
This is the DAG equivalent of:
https://reviews.llvm.org/rL284649
plus the fold for the NUW case which already existed in InstSimplify.
Note that we miss a vector fold because of a deficiency in the DAG version of
computeKnownBits().
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284844 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed